Catalytic reforming is a process with C6-C12 naphtha fractions as feedstocks for producing high-octane gasoline, aromatics and hydrogen by subjecting the feedstock hydrocarbon molecules to reforming reactions such as dehydrogenation of cycloalkane, dehydroisomerization of straight chain alkane and dehydrocyclization of paraffins, etc. in the presence of hydrogen and catalysts at a certain temperature and a pressure. A supported bifunctional reforming catalyst widely adopted in current catalytic reforming technology comprises the hydrogenation/dehydrogenation function provided by a metal component and the acidic isomerization function provided by a support. The reforming catalyst is typically a bi(or multi)metallicatalyst using active alumina as the support and Pt as the major metal component, and comprising a second metal component such as rhenium, tin or germanium.
As for the bifunctional reforming catalyst, the metallic function and the acidic function act synergistically on the catalytic reforming reaction in a certain matching degree. If the hydrogenation/dehydrogenation active function of the metal is too strong, carbon deposit on surfaces of the reforming catalyst will increase rapidly, which goes against the proceeding of the reforming reaction; and if the function of the metal is too weak, the activity of the catalyst will decrease. If the acidity is too strong, the hydrocracking activity of the catalyst is comparatively strong and the liquid yield of the reforming product will decrease; and if the acidity is too weak, the activity will decrease. Therefore, the balanced match between the acidic function and metallic function of the support determines the activity, selectivity and stability of the catalyst.
In addition, as for a platinum-rhenium reforming catalyst, since the metal rhenium has a quite high hydrogenolysis activity, if the activity of rhenium is not passivated at the beginning of operation, a drastic hydrogenolysis reaction will occur in the initial state of feed supply, which releases a great amount of reaction heat to make the temperature of the catalyst bed rise rapidly and cause an overtemperature phenomenon. Once such a phenomenon occurs, serious consequences tend to be caused. Minor consequences include a large amount of carbon deposit of the catalyst, which decreases the activity and stability of the catalyst; and serious consequences include burning out the catalyst, reactor and internal components. Hence, the platinum-rhenium reforming catalyst needs to be presulfied before feedstock injection. The excessive hydrogenolysis reaction of a fresh catalyst is reduced through presulfurization so as to protect the activity and stability of the catalyst and improve the selectivity of the catalyst. Methods for presulfarization of the platinum-rhenium catalyst include two types, one of which introduces H2S into hydrogen and carries out presulfurization of the catalyst slowly under temperature and pressure, and the other of which injects organic sulfides such as dimethyl disulfide and dimethyl sulfide and so on into hydrogen under certain temperature and pressure and uses H2S formed after decomposition of these organic sulfides for presulfurization of the catalyst. The first method is usually used in laboratory investigation and the second method is widely used for a start-up of industrial devices of the platinum-rhenium catalyst. These two methods both have the nature of presulfurizing the catalyst with H2S and both pertain to gas-phase sulfurization. The presulfurization of the platinum-rhenium reforming catalyst has problems of equipment corrosion, environmental pollution and security risks and the like.
Sulfate ions in the reforming catalyst are generally considered to hurt performances of the catalyst and are poisons to the catalyst. CN98117895.2 discloses a method of removing sulfate ions from the reforming catalyst by introducing organic chlorine compounds, which are decomposed into hydrogen chloride in presence of hydrogen at 400° C. to 600° C., into the catalyst bed poisoned with sulfate ions so as to remove them. This method can effectively remove the sulfate ions on the catalyst compared with the conventional regeneration of the catalyst by an oxychlorination process.
CN 102139221B discloses a platinum-rhenium reforming catalyst comprising 0.1 to 0.3% by weight of sulfate ions. Said sulfate ions are introduced through a co-impregnation or separate-impregnation method during the preparation of the catalyst. The catalyst as obtained can contact the reforming feedstock for the reforming reaction without presulfurization step. The catalytic performance and stability are improved and the start-up operation procedure is simplified.